Onboard automatic tire inflation systems are currently available for use on trucks and other vehicles. Such systems have been successfully used in non-powered vehicle axle assemblies (i.e., the axle assemblies of trucks or other vehicles which do not include powered drive shafts linked to the vehicle differential or other drive system) to deliver make-up air to a vehicle tire in the event that the tire is punctured or begins to leak or for other reasons, e.g., controlling the tire pressure based on road conditions.
By way of example, an onboard system for maintaining a predetermined pressure in each of the tires of a non-powered truck tandem axle assembly is currently available from Airgo, Inc. of Edmond, Okla. The Airgo system delivers compressed air from an onboard compressor (e.g., the compressor used for supplying air to the truck brakes) to the interior of the tandem axle, or to a tube extending through the axle, in the event that a leak occurs in any of the four tandem axle tires. The Airgo system also includes: a pair of rotary seals provided proximate the outer ends of the non-powered tandem axle; a set of air lines extending from the rotary seals for delivering air from the interior of the axle, through the rotary seals, to each of the four tires; check valves provided in the air lines for preventing reserve air flow from the tires to the axle; and an indicator light which alerts the operator that a leak has developed. The system controls the make-up air flow in accordance with the operating pressure required by the tires. For most tandem truck axles, the automatic inflation system will typically be operable for providing a sufficient make-up air flow to maintain a tire pressure of at least 90 psig and more preferably at least 95 psig. An onboard automatic inflation system of this type is described, for example, in U.S. Pat. Nos. 6,105,645 and 7,418,892, the entire disclosures of which are incorporated herein by reference.
Although the rotary union assembly employed in the Airgo system allows the use of onboard automatic tire inflation systems in non-powered axle assemblies, the development of a commercially viable system which would allow the use of such onboard automatic tire inflation systems in the powered drive axle assemblies of trucks and other vehicles proved to be more problematic due to the presence of the rotating axle within the spindle. Commercially viable systems for non-powered axle assemblies were not adaptable for use on drive axles and the available drive axle systems usable on other types of vehicles had significant shortcomings and disadvantages. For example, such systems (a) would typically inflate only during a portion of the revolution of the tire or only when the vehicle was stationary, (b) could not operate at highway speeds, (c) did not provide continuous seal lubrication, (d) were not self contained, and/or (e) required external components which did not fit with the existing axle assembly.
Subsequently, Airgo, in cooperation with Oklahoma State University developed a new rotary union assembly for use in an automatic tire inflation system for powered drive axle assemblies which is the subject of U.S. Pat. No. 7,896,045, the entire disclosure of which also is incorporated herein by reference. While the new rotary union assembly operated as intended, it was comprised of multiple components, increasing its cost of manufacture and making it difficult to assemble and susceptible to leakage. The assembly also introduced air into the rotary union in an axial direction as was typical of the available rotary union assemblies for non-powered axle assemblies. The force of the axially directed incoming air tended to push outwardly on the rotary union and caused the components thereof to tend to separate in the axial direction which could result in the loss of the seal between the rotating and non-rotating components. This issue became more significant in the event of off center wheel mountings due to the resulting wobble of the rotating portions of the assembly with respect to the stationary portions. The combination of wobble and high pressure axial air flow can cause the seal to periodically disengage from its contact surface increasing the possibility of leakage. To reduce the wobble caused by off center mountings, customized spacers were employed for each axle variation, due to the inconsistencies in the distance from the front of the spindle to the back of the axle plate. Development efforts continued and the aforesaid shortcomings with the prior drive axle system have been obviated by the present invention.